Liquid Reductant Dosing Module with Heating Device

ABSTRACT

A liquid reductant dosing module for a combustion exhaust system is disclosed comprising an enclosed reservoir comprising a top, a bottom, one or more sides, an inlet, and an outlet. A resistive wire rod heater is disposed in that reservoir, comprising a hollow member extending along a first axis between the bottom of the reservoir and the top of the reservoir. Along that member there is disposed resistive metal wire such that when electric current is applied to the resistive metal wire, a greater portion of electric power is distributed as heat proximate to the bottom of the reservoir than is distributed as heat proximate to the top of the reservoir.

BACKGROUND OF THE INVENTION

This invention relates to a reservoir for a fluid dosing system. Morespecifically, the invention relates to a reservoir for holding areducing agent for introduction into a combustion exhaust gas.

The emission of nitrogen oxide (NO_(x)) compounds in engine exhausts haslong been the focus for health professionals and regulatory agenciesworldwide. In many locations, regulations require stringent reductionsof NO_(x) levels in new equipments. NO_(x) emissions may be found in avariety of systems such as internal combustion engines, gas turbineexhaust, lean burn engines, industrial boilers, process heaters or otherprocess streams.

In order to reduce NO_(x) emissions, it is known to use a selectivecatalytic reduction (SCR) device to treat an exhaust flow and tosignificantly reduce NO_(x) emissions. In an SCR system a reducingagent, for example urea solution, is dosed into the exhaust gas flowupstream of an SCR catalyst. This reducing agent is then usually reactedin the presence of a catalyst downstream of the injection point in anSCR device. Within the SCR device NO_(x) compounds are then reduced tonitrogen. WO2004111401 discloses such a device.

The general operation of an SCR device is shown in FIG. 1, in which adiesel engine 1 produces an exhaust flow comprising various exhaustgases 3. The exhaust gases are conveyed through an exhaust system,indicated generally at 5, comprising an oxidation catalyst device 7, aselective reduction catalyst device 9 and a slip catalyst 11.

The oxidation catalyst device 7 is a flow through device that consistsof a canister containing a honeycomb-like structure or substrate. Thesubstrate has a large surface area that is coated with an activecatalyst layer. This layer contains a small, well dispersed amount ofprecious metals such as platinum or palladium. As the exhaust gasestraverse the catalyst, carbon monoxide, gaseous hydrocarbons and liquidhydrocarbon particles (unburned fuel and oil) are oxidized, therebyreducing harmful emissions.

The SCR device 9 performs SCR treatment of NO_(x) using ammonia derivedfrom a source of urea as a chemical reductant. A slip catalyst 11 may belocated downstream of the SCR device 9 to clean up any unreactedammonia.

Urea for the SCR device 9 is stored in a tank 13 which is in fluidcommunication with the exhaust system 5. A pump 15 is provided to pumpurea from the tank 13 to the exhaust system 5. The supply of urea iscontrolled by a control unit 17, for example the engine control unit,which receives engine speed and other engine parameters from the engine1. An injection device 19 (also referred to herein as a fluid dosingdevice) is used to inject the urea into the exhaust flow.

As a 32.5% urea solution freezes at −11.5° C., urea delivery systemsmust be adapted for delivery of liquid urea to the vehicle exhaustsystem under conditions that would normally cause the liquid urea tofreeze. One solution would be to simply heat the storage tank 13.However, this can require substantial quantities of energy to maintainthe entire storage tank 13 in a liquid state and can take significanttime to thaw if the tank has become completely frozen. An alternativearrangement is to place a smaller reservoir downstream of the storagetank that can be unfrozen quickly and/or maintained as liquid moreefficiently since it contains a smaller amount of liquid urea. Such anarrangement is shown in FIG. 2, with urea flowing from storage tank 23into dosing reservoir 33 through inlet 25, and then pumped out of dosingreservoir 33 by pump 15 through outlet 27 from where it flows to theexhaust stream as shown in FIG. 1. Alternatively, the dosing reservoircan be positioned adjacent to and in physical contact with the storagetank or even inside the storage tank so that heat from the heated dosingreservoir during prolonged periods of operation will help thaw thestorage tank or maintain it in a liquid state.

In any case, a liquid reductant reservoir (whether it is storage tank 13or smaller dosing reservoir 33) will need to be heated in order toprovide liquid reductant to the exhaust system. One proposed approachhas been to use a submerged ceramic PTC heater in the reservoir.However, this approach provides heat at the bottom of the reservoir, butnot at the top. As liquid reductant is dosed into the exhaust system,the level in the reservoir drops, resulting in a cavity of dead airspace forming between the frozen reductant toward the top of thereservoir and a level of liquid underneath, which can result in poorheat transfer to the remaining frozen urea. Further, as the liquid urealevel continues to drop, the ceramic PTC heater element may itselfbecome exposed to air, at which point its self-regulating heatingfunction will result in restricted power to the heater for furthermelting of frozen urea.

An alternative heater approach is to use a vertical ceramic PTC rodheater that runs from the top to the bottom of the urea reservoir. Thisapproach can provide good heating near the top of the reservoir, but maynot heat the bottom quickly enough to provide liquid urea for exhausttreatment right after vehicle startup. Also, as the urea level dropsduring dosing, exposure of the top of the ceramic PTC rod heater to theair can result in power reduction due to the heater's self-regulatingfunction.

Therefore, there is a need in the art for providing heat to a reservoirfor use in a dosing system that addresses the above mentioned problems.

SUMMARY OF THE INVENTION

Therefore, according to the present invention, there is provided adosing module for a combustion exhaust system comprising an enclosedreservoir comprising a top, a bottom, one or more sides, an inlet, andan outlet. A resistive wire rod heater is disposed in that reservoir,comprising a hollow member extending along a first axis between thebottom of the reservoir and the top of the reservoir. Along that memberthere is disposed resistive metal wire such that when electric currentis applied to the resistive metal wire, a greater portion of electricpower is distributed as heat proximate to the bottom of the reservoirthan is distributed as heat proximate to the top of the reservoir.

The above-described dosing module provides greater heat at the bottom ofthe reservoir for rapid melting of liquid reductant to be dosed to anexhaust system while still providing some power (or at least conductanceof heat) to the top of the reservoir to melt any residual frozenreductant. These and other advantages and features will become moreapparent from the following description taken in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 depicts a known SCR system.

FIG. 2 depicts a known reservoir configuration for dosing liquidreductant as part of an SCR system.

FIG. 3 shows an exemplary liquid reductant dosing module according tothe invention with a heat conductor attached at one end of the heater.

FIGS. 4A and 4B show an exemplary heater and pickup tube assembly for aliquid reductant dosing module according to the invention with a heatdeflector assembly.

DETAILED DESCRIPTION

Referring now to the Figures, the invention will be described withreference to specific embodiments, without limiting same. Turning now toFIG. 3, there is shown dosing reservoir 33 having a heater thereinaccording to an exemplary embodiment of the invention. The heater isshown in cut-away fashion in FIG. 3 with hollow tube 32 having windingsof resistive metal wire 39 thereon. The resistive metal wire may be oneither the inner surface or the outer surface of tube 32. In oneexemplary embodiment, it is on the inner surface, which may provideprotection against damage to the wire. The tube 32 may be of anymaterial that is compatible with the reductant composition in thereservoir and the temperature of the wire windings 39 when heated,although in one exemplary embodiment it is a heat-conductive metal suchas stainless steel. The resistive metal wire 39 may be of any metalhaving a resistance in the range that renders it useful for a heatingelement in this application, which in an exemplary embodiment is in therange of 1 ohm to 1.5 ohm. The total length of the resistive metal wire39 depends on the configuration of the reservoir and environmentalconditions. In one exemplary embodiment, the total wire length rangesfrom 1 to 1.5 meters.

The dosing reservoir also includes a liquid reductant pickup tube 31 fordrawing liquid reductant from near the bottom of reservoir 33 anddischarging out the top to the vehicle exhaust. Pickup tube 31 may be ofa suitable material, and may be of the same material as hollow tube 32,e.g., stainless steel. The pickup tube is shown in FIG. 3 as beinginside the hollow tube 32, which can help to ensure that it is quicklythawed by the heat from the resistive metal wire 39, but the pickup tube31 can also be positioned adjacent to the tube 32 or anywhere else inthe reservoir 33.

The resistive metal wire 39 is configured so that when electric currentis applied to it, a greater portion of electric power is distributed asheat proximate to the bottom of the reservoir than is distributed asheat proximate to the top of the reservoir. One way this can beaccomplished is by placing a greater density of metal wire toward thebottom of the tube 32 than toward the top of tube 32 and applying avoltage differential between an electrical leads 40 at opposite ends ofthe wire, shown in FIG. 3 to both be at the top end of the tube 32. Inan exemplary embodiment of the invention, the distribution of electricpower is broken down into multiple zones, and FIG. 3 depicts three suchzones: a low power density zone 34 proximate to the top of tube 32 wherethe spacing of the courses as the wire winds along the tube 32 arerelatively far apart, a high power density zone 36 proximate to thebottom of tube 32 where the spacing of the courses as the wire windsalong the tube 32 are relatively close together, and a medium powerdensity zone 35 in between zones 34 and 36 where the spacing of thecourses as the wire winds along the tube 32 is between the spacing forzone 34 and the spacing for zone 36. The proportion of the length oftube 32 taken up by each of the zones 34, 35, and 36 may vary dependingon design parameters. In one exemplary embodiment, each of zones 34, 35,and 36 covers approximately one third of the total length of tube 32.The distribution of power along the tube 32 can vary depending on theconfiguration of the reservoir, position of the heater, etc., but thepower in zone 36 should be sufficiently high to rapidly melt frozenreductant upon a cold vehicle start while the power in zone 34 should besufficiently low to avoid overheating when the level of liquid reductantin the reservoir drops before the storage tank melts sufficiently torefill it. In one exemplary embodiment it is 0-1 watts/in² of the tubesurface area along the upper zone of tube 32, 2.5-3.5 watts/in² alongthe middle zone of tube 32, and 8-10 watts/in² along the bottom zone oftube 32. In an embodiment where there is zero power distribution alongthe upper zone of tube 32, the upper electric lead attaches at theinterface of zones 34 and 34 instead of at the top of zone 34, and thewire in zone 34 acts only as a conductor of heat generated in thepowered portions of the wire.

In an alternate exemplary embodiment of the invention, the powerdistribution profile along the tube 32 is achieved through the use ofmaterials of different resistivities that have been electricallyconnected to one another. For example, in the three-zone embodimentshown in FIG. 3, the wire in zone 36 would have a higher resistivitythan the wire in zone 35, which would have a higher resistivity than thewire in zone 34. In this embodiment, zone 34 could still be given zeropower by positioning the upper lead at the interface of zones 34 and 34and having the wire in zone 34 functioning only as a heat conductor, inwhich case its electrical resistivity is not relevant since it is notpart of the electric circuit. In an exemplary implementation of thisembodiment, the wire in zone 35 has a resistivity of 0.25 ohm-m to 0.5ohm-m and the wire in zone 36 has a resistivity of 1.5 ohm-m to 2.5ohm-m, and the wire in zone 34 has a resistivity of 0 ohm-m to 0.25ohm-m. Resistivity may be varied by choosing different materials havingdifferent resistivities and/or varying the wire diameter.

In yet another alternate exemplary embodiment, the power distributionprofile along the tube 32 can be achieved through the use of separatecontrol circuits for different zones along the tube 32. In thisembodiment, each zone may have its own set of electric leads connectedto an independent controller and/or control circuit so that the power toeach zone is directly and independently regulated by a controller and/ora control circuit to provide a greater distribution of power proximatethe bottom of tube 32 than proximate the top of tube 32.

The various alternate embodiments for achieving a power distributionprofile may be used independently or in combination. For example,sections of wire may have different winding densities and be made ofdifferent material, or zones of wire connected to independent controlcircuits may be made of materials having different resistivities, or allthree embodiments may be used in combination.

In yet another exemplary embodiment of the invention, the resistivemetal wire 39 has a positive temperature coefficient of resistivity(TCR) that enables self-regulation of the temperature of the wire for agiven voltage applied to it. Wire with a positive TCR will exhibitgreater levels of resistivity at greater temperatures and lower levelsof resistivity at lower temperatures. In this case, as temperature ofthe wire starts to rise, either because liquid reductant level in thereservoir has dropped causing a portion of the wire to be exposed to airor because the liquid reductant itself has been heated, the resistivitystarts to rise so that for a given voltage, current flow through thewire is regulated and less heat will be generated. In an exemplaryembodiment, the resistive metal wire 39 has a TCR ranging from 0.1ohm-m/° C. to 0.15 ohm-m/° C. In an alternate embodiment, or inconjunction with the use of positive TCR wire, this type ofself-regulation can be achieved and/or enhanced by controlling thecurrent supplied to the resistive metal wire with a Wheatstone bridgecontrol circuit to control current to the heater based on the resistancethat it sees applied across the bridge. For example, when used withresistive metal wire that may have a positive TCR, but where the TCR isnot large enough to provide self-regulation on its own, a Wheatstonebridge, which will see increasing resistivity of the resistive metalwire 39 as temperature of the wire increases, will reduce the currentprovided to the resistive metal wire 39, thereby enhancing thetemperature regulation of the wire.

With continuing reference to FIG. 3, there is shown an exemplaryembodiment where there is an annular-shaped heat conductor 38surrounding tube 32, having heat-conducting fins 37 extending radiallytherefrom. The heat conductor 38 and heat-conducting fins 37 can be madefrom any heat-conducting material such as aluminum or stainless steel.In an exemplary non-limiting embodiment, the heat conductor 38 andheat-conducting fins 37 are effective to reduce the effective powerdensity in zone 36 to 0.5 to 1.5 watts/in². Materials such as aluminum,which are susceptible to reaction with reductants like urea, may be havea thin layer to provide protection from the reductant, such as apolytetrafluoroethylene coating or a layer of anodized aluminum.

The heated liquid reductant dosing reservoir according to the inventionprovides more heat toward the bottom of the reservoir where it isneeded, and that heat can also be advantageously carried to otherportions of the reservoir by convection. In some situations, however,because the pickup tube 31 extends below the bottom of tube 32, it maybe desirable to provide more heat below the bottom of the tube 32 tothaw or maintain liquid at the inlet of the pickup tube 31. One way toprovide such heat distribution is with a deflector. The deflector servesto deflect toward the reservoir bottom some of the convective heattransfer that would normally go from the bottom of the reservoir to thetop of the reservoir, thus forcing more heat at the bottom near theinlet of pickup tube 31. In one exemplary embodiment, the deflector maybe a simple cup-shaped member with a hole in the bottom of the cup inwhich the tube 32 would be disposed with the cup-shaped member inverselymounted near the bottom of tube 32. An alternative exemplary deflectordesign is shown in FIGS. 4A and 4B. The deflector assembly is shown inan exploded view in FIG. 4A, with bottom piece 44 having side openings45, bottom openings 46, and central opening 47, and a top piece havingcollar portion 43 adapted to fit around tube 32 and flange portion 42having openings 41. Openings 41, 45, and 46 may optionally be fittedwith a filter media such as a stainless steel mesh filter (not shown).The deflector is assembled with the tube 32 engaging bottom piececentral opening 47 and top piece collar 43. The upper edge of bottompiece 44 is joined to the outer periphery of the upper piece flangeportion 42 by means known in the art, such as crimping, welding, gluing,and the like.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description.

1. A liquid reductant dosing module for a combustion exhaust treatmentsystem comprising: (a) an enclosed reservoir comprising a top, a bottom,one or more sides, an inlet, and an outlet; and (b) a resistive wire rodheater disposed in said reservoir, comprising: (1) a hollow memberextending along a first axis between the bottom of said reservoir andthe top of said reservoir; (2) resistive metal wire disposed along saidmember such that when electric current is applied to said resistivemetal wire, a greater portion of electric power is distributed as heatproximate to the bottom of said reservoir than is distributed as heatproximate to the top of said reservoir.
 2. A liquid reductant dosingmodule according to claim 1 wherein there is a greater density ofresistive metal wire disposed along said member proximate to the bottomof said reservoir than is disposed along said member proximate to thetop of said reservoir.
 3. A liquid reductant dosing module according toclaim 1 wherein resistive metal wire disposed along said memberproximate to the bottom of said reservoir has a greater resistivity thanresistive metal wire disposed along said member proximate to the top ofsaid reservoir.
 4. A liquid reductant dosing module according to claim 1wherein resistive metal wire is disposed along said member such thatwhen current is applied to said resistive metal wire, 0 to 10 percent ofthe heat generated by such current is distributed along a zone of saidmember proximate to the top of said reservoir, 60 to 80 percent of theheat generated by such current is distributed along a zone of saidmember proximate to the bottom of said reservoir, and 20 to 30 percentof the heat generated by such current is distributed along a zone ofsaid member between the zone proximate to the top and the zone proximateto the bottom of said reservoir.
 5. A liquid reductant dosing moduleaccording to claim 1 wherein resistive metal wire is disposed along saidhollow member such that when current is applied to said resistive metalwire, 0 to 1 watts/in² is distributed along a zone of said hollow memberproximate to the top of said reservoir, 8 to 10 watts/in² is distributedalong a zone of said hollow member proximate to the bottom of saidreservoir, and 2.5 to 3.5 watts/in² is distributed along a zone of saidmember between the zone proximate to the top and the zone proximate tothe bottom of said reservoir.
 6. A liquid reductant dosing moduleaccording to claim 1 wherein said inlet is located proximate to thebottom of said reservoir.
 7. A liquid reductant dosing module accordingto claim 1 wherein said outlet comprises a pickup tube extending along asecond axis, which may be the same as or different than said first axis,extending between the bottom of said reservoir and the top of saidreservoir, said pickup tube having an open end proximate to the bottomof said reservoir.
 8. A liquid reductant dosing module according toclaim 7 wherein said pickup tube is disposed inside said hollow member.9. A liquid reductant dosing module according to claim 1 wherein saidhollow member is stainless steel.
 10. A liquid reductant dosing moduleaccording to claim 4 wherein the resistivity of the resistive metal wirealong a zone of said member proximate to the top of said reservoir is 0ohm-m to 0.25 ohm-m, the resistivity of the resistive metal wire along azone of said member proximate to the bottom of said reservoir is 1.5ohm-m to 2.5 ohm-m, and the resistivity of the resistive metal wirealong a zone of said member between the zone proximate to the top andthe zone proximate to the bottom of said reservoir is 0.25 ohm-m to 0.5ohm-m.
 11. A liquid reductant dosing module according to claim 1 whereinsaid wire material has a resistivity of 1 ohm-m to 1.5 ohm-m.
 12. Aliquid reductant dosing module according to claim 2 wherein said wirematerial has a resistivity of 1 ohm-m to 1.5 ohm-m.
 13. A liquidreductant dosing module according to claim 1 wherein said wire materialhas a positive temperature resistivity coefficient ranging from 0.1ohm-m/° C. to 0.15 ohm-m/° C.
 14. A liquid reductant dosing moduleaccording to claim 1, further comprising a Wheatstone bridge controlcircuit to control the current applied to said resistive wire.
 15. Aliquid reductant dosing module according to claim 1, further comprisinga heat sink member in thermal contact with said heater in said zoneproximate to the bottom of said reservoir.
 16. A liquid reductant dosingmodule according to claim 5, further comprising a heat sink member inthermal contact with said heater in said zone proximate to the bottom ofsaid reservoir such that when current is applied to said resistive metalwire, 0.5 to 1.5 watts/in² is distributed through the combined surfacearea of said heat sink member and said hollow member along the zone ofsaid hollow member proximate to the bottom of said reservoir.
 17. Aliquid reductant dosing module according to claim 1, further comprisinga deflector to inhibit convectional movement of liquid reductant fromproximate the bottom of said reservoir toward the top of said reservoir.18. A liquid reductant dosing module for a combustion exhaust treatmentsystem comprising: (a) an enclosed reservoir comprising a top, a bottom,one or more sides, an inlet, and an outlet; (b) a rod heater disposed insaid reservoir extending along a first axis between the bottom of saidreservoir and the top of said reservoir; and (c) a heat deflector memberdisposed circumferentially around said rod header proximate to thebottom of said rod heater, configured to inhibit upward convective heattransfer in said liquid reductant.
 19. A liquid reductant dosing moduleaccording to claim 18 wherein said inlet is located proximate to thebottom of said reservoir.
 20. A liquid reductant dosing module accordingto claim 18 wherein said outlet comprises a pickup tube extending alonga second axis, which may be the same as or different than said firstaxis, extending between the bottom of said reservoir and the top of saidreservoir, said pickup tube having an open end proximate to the bottomof said reservoir.
 21. A liquid reductant dosing module according toclaim 20 wherein said pickup tube is disposed inside said hollow member.22. A liquid reductant dosing module according to claim 18 wherein saidheat deflector member comprises a top member having an opening thereindisposed circumferentially around said hollow member and at least oneside member attached to and extending downward from said top member suchthat there is a space between said hollow member and said at least oneside member.
 23. A liquid reductant dosing module according to claim 22,further comprising a bottom member having an opening therein disposedcircumferentially around said hollow member and attached to said atleast one side member.
 24. A liquid reductant dosing module according toclaim 23 wherein at least one of said top member, said at least one sidemember, and said bottom member has at least one opening therein throughwhich liquid reductant can pass.
 25. A liquid reductant dosing moduleaccording to claim 24 wherein said at least one opening through whichliquid reductant can pass has a filter medium disposed therein.